Folding of the recombinant human thyrotropin (TSH) receptor extracellular domain: identification of folded monomeric and tetrameric complexes that bind TSH receptor autoantibodies.

We have analyzed protein folding and disulfide bond formation in the extracellular domain of the human TSH receptor (hTSHR-ecd) expressed in Escherichia coli. This domain, which begins at the amino-terminus and ends at residue 415, is a major autoantigen in human autoimmune thyroid disease. Refolding of reduced and denatured hTSHR-ecd occurred in polyacrylamide gels treated with 0.25 M KCl for visualization of protein bands. Under conditions of partial renaturation, at least three forms of the hTSHR-ecd were resolved by reelectrophoresis using sodium dodecyl sulfate-polyacrylamide gel electrophoresis: 1) unfolded monomers, 2) folded monomers, and 3) tetramers. Disulfide bond formation was implicated in both folding and tetramerization, as reduction of these forms produced only unfolded monomers. A natural variant of the hTSHR (v1.3), sharing 231 N-terminal amino acids with hTSHR-ecd, formed folded monomers and dimers on renaturation, but not tetramers, implicating one or more of the five cysteine residues residing between positions 231-415 in the association of dimers into tetramers. Binding of three different sources of hTSHR antibodies to these various forms of the hTSHR-ecd was assessed by immunoblotting using: 1) murine monoclonal antibodies (MAbs) generated against hTSHR-ecd, 2) rabbit polyclonal antisera generated against overlapping synthetic peptides spanning residues 37-71 of the hTSHR-ecd, and 3) human immunoglobulin G from patients with Graves' disease and detectable hTSHR-Ab. One of the MAbs, shown to recognize residues 21-35, and the rabbit polyclonal antibodies bound to all three forms of the hTSHR-ecd. Some of the hTSHR autoantibodies bound predominantly to the monomeric forms of the hTSHR, but autoantibodies were also identified that recognized tetrameric hTSHR-ecd. These data demonstrate that hTSHR-Abs recognize differing nonlinear and linear epitopes in the hTSHR-ecd and provide a methodology that should be useful for further defining the structural requirements for folding of this functionally and immunologically important domain of the hTSHR.